Deutsche Auslegeschrift No. 1,154,548 discloses an electric switch wherein an electronegative, fluorine-containing gas, particularly sulfur hexafluoride (SF.sub.6) is used as the quenching and insulating medium. The known switch includes two contacts which are movable relative to each other and between which an arc is drawn when the circuit is opened. The contacts consist of carbon, preferably graphite, in order to keep the destruction of the contacts and the formation of decomposition products low during the active time of the arc. The contacts, which consist of graphite, are made solid and are inserted into cup-shaped electrodes, the wall of the cup-shaped electrodes engaging at least partially in undercuts of the contacts on the cylinder surface, so that a secure mechanical and electrical connection is obtained.
In high-voltage power circuit breakers, particularly compressed-gas breakers, it is advantageous to make the contacts hollow for the removal of the arc gases produced during the interrupting process.
U.S. Pat. No. 3,801,764 discloses an arrangement wherein the annular graphite electrode, configured for the removal of switching gases is enclosed with an annular, electrically conducting body with a friction fit, which, in turn, is connected with the corresponding contact piece in an electrically conducting manner. The intermediate body, which in this arrangement holds the electrode can be chosen so with respect to the currents and the material composition so that advantageous expansion coefficients are obtained as compared to the graphite electrode which, because of the arc, must take up high thermal stresses.
The last-mentioned arrangement requires a press fit in which the graphite nozzle as an insert is deep-cooled relative to the enclosing annular metal body while the enclosing annular body is brought to a high temperature.
It is desirable to make the contact piece supporting the graphite electrode of an electrically highly conducting material, particularly copper, in order to be able to transmit large rated currents without a significant temperature rise. However, the use of copper as the holder for graphite electrodes presents difficulties because due to the necessary excess dimensions of the friction mounting, the strength of the copper may be exceeded. Such a shrink joint is therefore not usable for the present case.
United States patent application, Ser. No. 366,541, filed June 4, 1973 dicloses a contact system for a high-voltage circuit breaker wherein the problem of joining the materials graphite and copper securely together is solved by pressing the nozzle body consisting of graphite cold into a tube which is made of cold-worked copper so that the nozzle body is enclosed by the tube with a friction fit.